linux-kernel - Re: [PATCH] kconfig: Only generate config_is

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Message-ID: <BANLkTikmO7D3kUgU62d4_VC2ZPZCnSEqKA@mail.gmail.com>
Date:	Wed, 18 May 2011 02:23:26 -0400
From:	Arnaud Lacombe <lacombar@...il.com>
To:	Sam Ravnborg <sam@...nborg.org>
Cc:	Michal Marek <mmarek@...e.cz>, linux-kbuild@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>,
	Valdis.Kletnieks@...edu
Subject: Re: [PATCH] kconfig: Only generate config_is_xxx for bool and
 tristate options

Hi,

[added Valdis.Kletnieks@...edu to CC:]

On Tue, May 17, 2011 at 3:53 PM, Sam Ravnborg <sam@...nborg.org> wrote:
> On Tue, May 17, 2011 at 05:35:32PM +0200, Michal Marek wrote:
>> For strings and integers, the config_is_xxx macros are useless and
>> sometimes misleading:
>>
>>   #define CONFIG_INITRAMFS_SOURCE ""
>>   #define config_is_initramfs_source() 1
>
> I'm late with this comment....
> Could we introduce "config_is_foo" using a syntax that
> does not break grepability?
>
> Maybe a syntax like this?
>
>    #ifdef CONFIG_FOO
>
> and
>
>    if (KCONFIG_FOO())
>
> Grepping for the use of a symbol is a very typical thing,
> so we should try to keep this.
> And with the suggested syntax above I expect fixdep to
> catch up both usage types.
>
Actually, there is already an issue, on a much smaller scale, in the
current tree with NUMA_BUILD and COMPACTION_BUILD. The real way to fix
this would be to always defines CONFIG_FOO, its value being 1 or 0
depending on whether or not the symbol is selected. This is a
+35k/-35k change.

Also, I find KCONFIG_FOO() is too specific to be in the kernel source,
and redundant with CONFIG_FOO.

I've been playing a bit with the preprocessor, and reached that point:

#define EXPAND(x)       __ ## x
#define CONFIGURED(x)   \
        ({  int __1 __maybe_unused = 1;         \
            int __ ## x __maybe_unused = 0;     \
            EXPAND(x); })

I am not specifically proud of that, use case would be:

extern func(void);
int fn()
{
        if(CONFIGURED(CONFIG_FOO))
                func();
}

The issue I am seeing is that it adds a dependency to the optimizer,
as gcc will not optimize the branch away at -O0.

The advantage is that it is not intrusive at all within kconfig.

 - Arnaud
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